Plasma-Assisted Epitaxial Growth of ZnSe Films In Hydrogen Plasma
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PLASMA-ASSISTED EPITAXIAL GROWTH OF ZnSe FILMS IN HYDROGEN PLASMA S. YAMAUCHI AND T. HARIU Department of Electronic Engineering, Sendai 980, Japan
Tohoku University
ABSTRACT Plasma-assisted epitaxy (PAE) was applied to the growth of ZnSe films on (100) GaAs for low temperature epitaxial growth in hydrogen plasma. High purity ZnSe films were successfully grown by the control of hydrogen gas flow rate and VI/II supply ratio. Hydrogen-chloride gas and nitrogen in gas mixed in pure hydrogen gas plasma around 2% respectively resulted 1 the growth of highly conductive n-type layers (630Scm- ) and nitrogenacceptor doped layers (N-acceptor level-100meV), however, the control of VI/II supply ratio is also very important for the efficient N-acceptor doping. The plasma optical emission spectroscopy indicated that Se-N is composed by the reaction of Ný-ion with SeH (or Se) in hydrogen and nitrogen mixed gas plasma with Se supply.
INTRODUCTION ZnSe has recently been extensively investigated in view of its applications to optoelectronic devices including the heterostructure with GaAso It has, however, been recognized that the growth of II-VI compound semiconductors with controlled electronic properties is much more difficult than IV and III-V materials due to the strong self-compensation effect by native defects or residual impurities, which are incorporated particularly at high growth temperatures. Thus non-thermal equilibrium growths at low temperatures by molecular beam epitaxy (MBE) (1), metalorganic chemical vapor deposition (MOCVD) (2), atomic layer epitaxy (ALE) (3) and so on have recently been attempted, aiming at the growth of purified ZnSe films and efficient doping. The purpose of this paper is to describe the plasma-assisted epitaxial growth of ZnSe films in hydrogen plasma, in order to extend the controllability of their electronic property. PAE APPARATUS The PAE apparatus developed for ZnSe growth is shown in Fig.l. Elemental Zn and Se shots were evaporated from quartz cells by resistive heating and supplied through gas plasma toward (100) GaAs substrates. Hydrogen gas (99.9999% pure gas was purified by palladium diffuser), sometimes mixed with HCl gas (99.95% pure) and N2 gas (99.9995% pure) for doping was introduced into the chamber and was excited through capacitive coupling by rf power at 13.56MHz. The substrate holder was settled parallel to the applied electric field to avoid the possible bombardment of high energy ions directly accelerated by the field. Optical emission from plasma was detected through a sapphire window at the bottom of the chamber. The detailed growth conditions have been described elsewhere (4). UNDOPED PAE-ZnSe GROWTH Typical photoluminescent (PL) spectrum of nominally undoped PAE-ZnSe films of 2.5Pm thickness grown at 3200C in hydrogen plasma is shown in Fig.2, where the strong emission due to free-exciton (Ex) is observed with weak deep level emissions. It is then concluded that the PAE-undoped ZnSe films are of high purity, however, it is to be noted here that the purity Mat. Res. S
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